1. Field of the Invention
The present invention relates to a piezoelectric transformer used in an inverter circuit for a back light of a cold cathode tube in a liquid crystal display panel for personal computers or car navigation systems, an adapter power source circuit used generally in electronic instruments for civil use, an electronic copier or the like, particularly to a structure thereof for mounting to a package.
2. Description of Related Art
In recent years development of a piezoelectric transformer using a piezoelectric effect has actively been performed as a voltage conversion element for generating high voltage used in a power source voltage in a device which needs high voltage, such as a back light inverter or a deflector in a television set. Compared with a conventionally used wiring type electromagnetic transformer, a piezoelectric transformer has numerous advantages such that downsizing and thinning thereof is facilitated, noninflammability thereof can be achieved since the piezoelectric transformer uses ceramics for its material, a noise caused by electromagnetic induction is not emitted, or the like.
With development of such a piezoelectric transformer, a highly reliable package for encapsulating a piezoelectric transformer is requested. A piezoelectric transformer generally comprises a drive unit and a power generating unit and is of a constitution where a total of the transformer is vibrated when an a.c. voltage is applied to the drive unit by which high voltage is generated in the generating unit (a case of step-up type transformer). Hence, a method of supporting such an oscillator becomes important in view of mounting the oscillator. Many reports have been issued in respect of methods of supporting such a kind of piezoelectric body in relation to a package structure concerning a piezoelectric resonator.
According to one of conventionally-known methods of supporting a piezoelectric body, there has been provided a method of forming projections to supporters. One example of such a method of supporting a piezoelectric body is disclosed in Japanese Unexamined Utility Model Publication No. JU-A-63-40018 (hereinafter, a first publication). FIG. 11(a) is a sectional view of a piezoelectric part disclosed in the first publication. FIG. 11(b) is a perspective view showing a support terminal in FIG. 11(a). Referring to FIG. 11(a), a piezoelectric element 1A is pinched by two support terminals 1013 and 1014 in a case 1012. A spherical protrusion 1013a as illustrated by FIG. 11(b) is formed in the support terminal 1013. Similarly, a protrusion 1014a is formed on the other support terminal 1014 at a position opposed to the projection 1013a. This piezoelectric part has a structure where the opposed projections 1013a and 1014a are brought into contact with the piezoelectric element 1A thereby supporting the piezoelectric element 1A.
However, according to the method of supporting the piezoelectric element described above, when it is applied to supporting, for example, a piezoelectric transformer, instantaneous disconnection may be caused at an electric terminal since the supporting bodies cannot follow the piezoelectric element when an impact is applied. It is conceivable as a means for solving such a problem to provide elastic spring performance to the supporter. The method of forming a bent portion in a supporter has been disclosed to realize such a structure.
An example of such a method is disclosed in Japanese Unexamined Utility Model Publication No. JU-A-4-85823 (hereinafter, referred to as a second publication). FIG. 12(a) is a sectional view of a piezoelectric resonance part according to the second publication. FIG. 12(b) is a front view showing a spring terminal in FIG. 12(a). Referring to FIG. 12(a), a piezoelectric element 1B in a flat plate shape is pinched by a projection 1023a of a spring terminal 1023 and a projection 1024a of a support terminal 1024 opposed to the spring terminal 1023, both of which are brought into contact with the piezoelectric element 1B. Specifically, the spring terminal 1023 has a structure where ridge-like spring portions 1023s are formed around the central portion of the terminal and the projection 1023a is formed at the central apex of the spring portions 1023s as shown by FIG. 12(b). Therefore, accordingly, a piezoelectric element 1B is supported by being pressed by the elastic spring performance of the spring terminal 1023 constituted as described above.
There have been other reports of examples where the elastic spring performance is provided by forming a bent portion in a supporter as in the spring portions 1023s in a ridge-like shape of the second publication. Technologies disclosed in Japanese Unexamined Utility Model No. JU-A-60-55117 (hereinafter, referred to as a third publication) or Japanese Unexamined Patent Publication No. JP-A-61-18209 (hereafter, referred to as a fourth publication) show examples of such a supporting method.
FIG. 13(a) is a sectional view showing a structure for holding a piezoelectric element according to the third publication. FIG. 13(b) illustrates a side view and a top view of a bent portion of a support terminal in FIG. 13(a). Referring to FIG. 13(a), a piezoelectric element 1C in a flat plate shape is pinched by a support element 1033 and a support element 1034 in a case 1032. A conductive elastic body 1035 is interposed between the support element 1034 and the piezoelectric element 1C. According to the support terminal 1034, a bent portion 1034a is formed at a portion thereof opposed to the elastic body 1035 whereby the spring elastic performance is provided and the piezoelectric element 1C is pressed and supported by the bent portion 1034a and the elastic body 1035. Incidentally, in respect of the bent portion 1034a, as illustrated by FIG. 13(b), the height is 1.5 through 3.0 times as large as the thickness T of the terminal and the width is 0.8 through 20.0 times as large as the width of the terminal.
A ceramic resonator according to the fourth publication is shown in FIGS. 14(a) and 14(b). FIG. 14(a) is a perspective view of a package and FIG. 14(b) is a sectional view of a mounted ceramic resonator. As shown by FIG. 14(a), a metal support plate 1043 is embedded in a case 1042. The support plate 1043 is constituted by two cut-off portions 1043b formed around the central portion and a spring-like portion 1043a formed by pushing out the cut-off portion. FIG. 14(b) shows the structure of the ceramic resonator encapsulating a piezoelectric element 1D by using two of the cases constituted as described above. As shown by FIG. 14(b), the piezoelectric element 1D is pressed and supported by the two spring-like portions 1043a and 1044a. There is other ceramic resonator using a similar supporting method disclosed in Japanese Unexamined Patent Publication No. JP-A-61-18208. According to the publication, a piezoelectric element is supported by support plates each formed by embedding an elastic material at a back face of a projection in contact with the piezoelectric element.
As examples for providing the elastic spring performance to supporters as disclosed in the above-described second through fourth publications, there have been known technologies of partially cutting and raising supporters. As examples of such supporting methods, there are utility models and inventions disclosed in Japanese Unexamined Utility Model No. JU-A-63-40019 (hereinafter, referred to as a fifth publication), Japanese Unexamined Utility Model Publication No. JU-A-63-136425 (hereinafter, referred to as a sixth publication) and Japanese Unexamined Patent Publication No. JP-A-4-35514 (hereinafter, referred to as a seventh publication).
FIG. 15(a) is a perspective view of a supporter used in a piezoelectric oscillation part according to the fifth publication. FIG. 15(b) is a side view showing a state where a piezoelectric element is supported by using the supporter of FIG. 15(a). Referring to FIG. 15(a), a leaf spring 1053 is formed by cutting and raising around the central portion of a supporter 1052. A support projection 1053a for supporting a piezoelectric element 1A by being brought into contact therewith is formed on the side of a front end of the leaf spring 1053. Further, projections 1054a and 1054b are formed in the proximity of the leaf spring 1053. As shown by FIG. 15(b), a piezoelectric element 1E is pressed and pinched by the supporter 1052 and a supporter 1055 where only a support projection 1056 is formed.
FIG. 16(a) is a broken perspective view of an electronic part according to the sixth publication. FIG. 16(b) is an enlarged perspective view of an elastic support piece in FIG. 16(a). Referring to FIG. 16(a), a piezoelectric element 1F is pinched by support terminals 1063 and 1064 in a case 1062. Projections 1063a and 1064a are formed respectively at central portions of the support terminals 1063 and 1064, and these portions are brought into contact with the piezoelectric element 1F. Further, elastic support pieces 1065 having the elastic spring performance which are formed by partially cutting and raising the portions of the support terminal 1064, are formed at the support terminal 1064 and the piezoelectric element 1F is pressed and held by the elastic support pieces 1065 and the support terminal 1063 having the elastic spring performance.
FIG. 17(a) is a sectional view showing the structure of a surface mounting quartz oscillator according to the seventh publication. FIG. 17(b) is a perspective view of a supporter in FIG. 17(a). Referring to FIG. 17(a), a quartz piece 1G in a flat plate shape is pinched by opposedly fixed supporters 1073 and 1074 in a case 1072. In respect of the supporter 1074, as shown by FIG. 17(b), a central portion of a flat plate made of a metal is cut and raised in a section-like shape thereby forming a leaf spring 1074a. The other supporter 1073 has a similar structure. Therefore, the quartz piece 1G is held by being pressed by the elastic spring performance of the supporters 1073 and 1074.
As an example of constituting a supporter by bending it as disclosed in the invention described in the seventh publication, there has been provided a technology disclosed in Japanese Unexamined Utility Model Publication No. JU-A-4-85824 (hereinafter, referred to as an eighth publication) although leaf springs are not used. FIG. 18 is a perspective view of a piezoelectric part described in the publication. Referring to FIG. 18, a piezoelectric substrate 1H in a flat plate shape is pinched by two input/output terminals 1082 front end portions of which are bent in a U-like shape at both end portions of the piezoelectric substrate 1H. A grounding terminal 1083 is arranged at the central portion of the piezoelectric substrate 1H. The front end portion of the grounding terminal 1083 is provided with the structure where the portion is bent and embedded into an outer case, not shown.
Also, there has been other technology disclosed in Japanese Unexamined Patent Publication No. JP-A-2-110811 (hereinafter, referred to as a ninth publication) as an example of utilizing a leaf spring although it is not concerned with a method of supporting a piezoelectric body. FIG. 19(a) is a sectional view showing the structure of a rotating magnetic head drum according to the ninth publication. FIG. 19(b) is a plane view of a grounding brush in FIG. 19(a). Referring to FIG. 19(a), static electricity generated in a rotating drum 1092 is escaped by a grounding brush 1091. As shown by FIG. 19(b), the grounding brush 1091 has a structure where a metal holding piece 1091a and a leaf spring 1091b overlaps each other and an end portion 1091c of the metal holding piece 1091a has a shape where the width is gradually tapered down.
Meanwhile, when the supporting methods of the second through the ninth publications are applied to supporting a transformer element in a piezoelectric transformer, the following problems are caused.
For example, according to each of the second, the fifth, and the sixth publications, only one of the two opposed supporters pinching a piezoelectric element, is provided with the elastic spring performance. The size of a piezoelectric transformer element is larger than those of piezoelectric elements (piezoelectric oscillators) used in the conventional technologies. Therefore, when vibration or impact is applied from a direction on the side of the supporter having no elastic spring performance, the supporter may be separated from the piezoelectric transformer element and instantaneous electric disconnection may be caused.
In contrast thereto, according to each of the third and the fourth publications, both of the two opposed supporters pinching piezoelectric elements, are provided with the elastic spring performance and therefore, the problem of generation of instantaneous electric disconnection is comparatively minor. However, an area thereof in contact with the piezoelectric element is very large, and therefore, when the structure is applied to an element having a large vibrating operation level such as a piezoelectric transformer element, a conversion efficiency between input and output is considerably deteriorated.
According to the method of supporting a quartz vibrator disclosed in the seventh publication, work shapes of parts are complicated, a number of parts is very large and assembling steps are large.
The supporter disclosed in the eighth publication naturally cannot follow vibration or impact from outside as in the first publication. In addition thereto, a total thereof is embedded in an outer case and accordingly, the vibration of the piezoelectric transformer is significantly hampered by which the conversion efficiency between input and output is considerably deteriorated.
Furthermore, according to the leaf spring disclosed in the ninth publication, although resonance is prevented by using two parts of the metal holding piece 1091a and the leaf spring 1091b, in this style, a number of parts must be increased to prevent resonance, which gives rise to an increase in cost.
A piezoelectric transformer encapsulated in a package as shown by FIGS. 20(a) and 20(b) and FIGS. 21(a) and 21(b), has been proposed in consideration of the problems of the conventional piezoelectric resonators, quartz oscillator and the like, which have been described as above. The piezoelectric transformer shown in FIGS. 20(a), 20(b), 21(a) and 21(b) is a piezoelectric transformer described in U.S. application Ser. No. 08/608060 filed by an assignee the same as that of the instant application.
FIG. 20(a) is an exploded perspective view of the piezoelectric transformer. FIG. 21(a) is a sectional view taken from a line A--A of FIG. 20(a). Referring to FIG. 21(a) and FIG. 21(a), a piezoelectric transformer element 1 in an elongated plate shape, is encapsulated in a space surrounded by an upper case 4A and a lower case 4B. A total of three surface electrodes of two input electrodes 52A respectively disposed on both end sides in the longitudinal direction and an output electrode 53A in a stripe shape disposed at the central portion thereof, are independently formed on the upper face of the transformer element 1. A total of the three surface electrodes are formed on the lower face of the transformer element 1 at respective regions opposed to the respective electrodes on the upper face thereof. Upper lead terminals 70A are extended from a side wall of the upper case 4A in the horizontal direction up to the centers of the input electrodes 52A on the upper side of the transformer element 1. A portion of the upper lead terminal 70A in the proximity of the font end thereof is bent in a V-like shape to the side of the transformer element 1 and is brought into contact with the input electrode 52A at a terminal portion 71A. Similarly, lower lead terminals 70B are extended horizontally from a side wall of the lower case 4B up to the central portion of the input electrode 52B on the lower face side of the transformer element 1 and the front end portion is bent and brought into contact with the input electrode 52B at a terminal portion 71B. The upper and lower lead terminals 70A and 70B each formed by working a flat plate starting material made of, for example, phosphor bronze plated with tin, is provided with the elastic spring performance and is embedded in the side wall of the upper and lower cases 4A or 4B to penetrate therethrough from inside to outside by an insert molding process. A portion of the input electrode extended outside of the side wall becomes the portion for connecting to outside. The upper lead element and the lower lead element having such a structure are also provided to the other one of the input electrodes of the transformer element 1. Further, the same are also installed to the output electrodes 53A.
Incidentally, the upper and lower cases 4A and 4B are provided with a snap fit structure one example of which is shown in FIG. 20(b). Therefore, the piezoelectric transformer can be assembled with certainty, simply and with no slack only by fitting together the two upper and lower cases 4A and 4B from top and from bottom.
When such a package is used, instantaneous electric disconnection is not caused between the electrodes on the surfaces of the transformer element 1 and the upper and the lower lead terminals even if impact or vibration is applied from outside since both of the upper and lower lead terminal 70A and 70B pinching the transformer element 1, are provided with the elastic spring performance. Further, as described in U.S. application Ser. No. 08/608060, when an area where the transformer element 1 and the respective upper and lower lead terminals 70A and 70B are brought into contact, is set to 0.5% or less of a projected area of an electrode forming face of the transformer element 1, for each of the terminals, the reduction in the conversion efficiency between input and output becomes substantially nonproblematic. Further, the piezoelectric transformer can be assembled only by mounting and fitting the transformer element 1 to the two upper and lower cases 4A and 4B and therefore, a number of parts and a number of assembling steps are small by which the reduction in cost is facilitated.
According to the piezoelectric transformer (refer to FIG. 20 (a) and FIG. 21(a)) described in the above-mentioned U.S. application Ser. No. 08/608060, initial dispersion and ageing are liable to change the state of contact between the electrodes on the surfaces of the transformer element and the lead terminals. As a result,
1 The initial dispersion and the ageing change in the electric conductive state are large and the reliability of conduction must be improved. PA1 2 An audible noise in the case where the piezoelectric transformer is encapsulated in a package is liable to cause initially and over time. An explanation will be given thereof as follows.
FIG. 21(b) is a side view showing a state (indicated by the broken lines in the drawing) where no load is applied on the lower lead terminal 70B by the piezoelectric transformer element 1 and a state (indicated by the bold lines in the drawing) where the lower lead element 70B is pushed down by the piezoelectric transformer element 1. When no load is applied on the lead terminal 70B, a terminal portion 71B is in parallel with the electrode forming face of the transformer element 1. Here, when the lead terminal 70B is pushed down by the transformer element 1, a lead terminal root portion 72B is bent with a side wall 5B of a case as a fulcrum as indicated by the bold lines by which a rotational angle is caused at the terminal portion 71B. The rotation is in a direction of rotating the terminal portion 71 B to the right in the drawing. Therefore, the terminal portion 71B is inclined to open toward the direction of extending the lead terminal in respect of the electrode forming face of the transformer element 1. Then, although the electrode on the side of the transformer element and the terminal portion of the lead terminal are originally brought into contact in a face contact, the contact becomes a line contact where the electrode is brought into contact with the piezoelectric element only at an edge line at the bent portion of the terminal portion 71B whereby the contact state becomes very unstable. Moreover, in consideration of mass production performance, the line contact between the electrodes of the transformer element and the terminal portions of the lead terminal, is deteriorated further into a point contact. That is, the lead terminal is fabricated by a method of punching out a starting plate material by a press mold and it is inevitable that burrs are caused at a peripheral portion of the lead terminal in the punching out operation. Further, with respect to a plated lead terminal, in addition to the adverse effect at the peripheral edge of the terminal portion 71B, deposition of plating is further created. That is, with respect to the contact between the electrode of the transformer element and the lead terminal, the contact state is deteriorated from the line contact at the edge line of the bent portion of the lead terminal to two points contact at the both ends of the edge line.
Meanwhile, it is difficult to control the sizes of the burrs or the deposition of plating and therefore, the contact state between the electrode and the lead terminal cannot be controlled even initially in the case of the above-described point contact. That is, only the contact where the dispersion of electric conductive state is very large, can be obtained. Meanwhile, in order to prevent the audible noise due to the lead terminal, the lead terminal is normally designed such that the natural frequencies of the bending mode and the torsion mode do not coincide with the natural frequencies of the bending mode and the torsional mode of the transformer element and the natural frequencies of the lead terminal do not coincide also with the drive frequency of the transformer element. However, in such a lead terminal having such a large dispersion, it is difficult to always satisfy, from the start of fabrication binding conditions of spring designed in such a way, and accordingly, an audible noise is liable to cause.
Further, in the state where the point contact is caused, the contact between the electrode on the side of the transformer element and the lead terminal is not smooth, and accordingly the electrode of the transformer element may be scratched or only a portion of the electrode may be chipped off deeply by the use over a long period of time by vibration or impact from outside. As a result, a noise may be caused by causing instantaneous disconnection due to jumping of the lead terminal, or the audible noise may be going to be emitted gradually during the time of using the lead terminal even if the audible noise is not caused initially. The electric conductive state is naturally deteriorated.